In general, a method of acquiring an X-ray image by injecting a contrast medium into the blood vessels is used for checking the state and position of blood vessels in the body. Further, in recent years, an angiography method such as a computed tomography (CT) angiography and a magnetic resonance angiography (MRA) by which a three-dimensional image can be acquired has also been developed.
Further, a method of imaging a flow path such as a blood vessel by using an optical method has also been proposed (see Patent Literature 1) in the past. In the imaging system described in Patent Literature 1, an interference light image is captured at a first timing by light that is reflected on an object and interfered, which is emitted from a light emitting unit, and a luminescent image is captured at a second timing by light emitted from the object.
As described above, in recent years, in the medical field and the like, various imaging methods using optical methods have been developed, and the detection accuracy thereof is also improved year by year. Here, in the imaging technology using an optical method, there is a concern that occurrence of various types of noise reduces the detection accuracy, and a speckle is known as one of such noise. The speckle is a phenomenon in which a spotty swaying pattern appears on an irradiation surface in accordance with the uneven shape of the irradiation surface. In recent years, technologies relating to a method of imaging a flow path such as a blood vessel by using a speckle that is one of such noise are also being developed.
Here, the speckle is a random interference/diffraction pattern due to scattering or the like in an optical path. Further, the magnitude of the speckle is represented by an index called a speckle contrast that is a value obtained by dividing the standard deviation of the intensity distribution by the average of the intensity distribution. When an object illuminated with coherent light is observed using an imaging optical system, a speckle due to scattering of the object is observed on the image surface. Then, when the object moves or the shape of the object changes, a random speckle pattern corresponding thereto is observed.
When a light scattering fluid such as blood is observed, the speckle pattern changes by the change in the fine shape due to flow as time passes. At that time, when an image sensor is placed on the image surface and a fluid is imaged with an exposure time sufficiently longer than the change of the speckle pattern, the speckle contrast of a part where blood is flowing, i.e., a part of blood vessels, is reduced by time averaging. By using such a change in the speckle contrast, angiography can be performed.